1. Field of the Invention
The present invention relates liquid isocyanates of methylene diphenylisocyanate (MDI) comprising isocyanate prepolymers of the MDI having urea and/or biuret groups in the prepolymer backbone which are prepared by reacting MDI of a specified composition with a select group of secondary amines.
2. Brief Description of the Prior Art
Urea and/or biuret containing isocyanate prepolymers are generally known in the art. U.S. Pat. No. 3,862,973 discloses biuret containing polyisocyanates which are prepared by reacting organic polyisocyanates with secondary amines at temperatures of from about 80 to 200 degrees Centigrade. Monoamines employed therein are selected from the group consisting of N-methyl aniline, N-ethylaniline, octadecylniline, diphenylamine, dibenzylamine, pyrrolidone, morpholine and the like. Polyamines employed therein are selected from the group consisting of N,N'-diethyl-4,4'-diamino-diphenylmethane, N,N'-diethyl-2,6-tolylene diamine, N,N'-diamino diphenylether and the like.
U.S. Pat. No. 4,618,706 discloses the process for the preparation of aromatic polyisocyanates containing urea and/or biuret groups, optionally in the form of a mixture of homologues and/or isomers by the reaction of aromatic diisocyanate with diamines having primary or secondary amino groups at about 20 to 180 degrees Centigrade.
Quite often, the preparation of these prepolymers entails the reaction of polyisocyanates with mono or polyamines at elevated temperatures. The use of high temperatures is uneconomical and can result in product discoloration, and in some cases the liberation of volatile and toxic by products via a series of biuret exchange reactions.
Several methods have been developed to overcome the problems associated with high temperatures. By the present invention there is provided a facile process for preparing liquid isocyanates containing urea and/or biuret groups.